1. Field
The present invention relates to a first transceiver unit, acting as a receiver unit with respect to a direction of communication, and a second transceiver unit, acting as a transmitter unit with respect to the same direction of communication,
said first transceiver unit comprising:
                a receiver adapted to receive from a physical channel a signal modulated over at least one carrier,        a channel analyzer coupled to said receiver, and adapted, while an initialization sequence is being transmitted over said at least one carrier, to determine a signal component and a noise component within a frequency interval enclosing one carrier out of said at least one carrier,        a first communication means adapted to establish a communication link with said second transceiver unit,        a power remote control unit coupled to said channel analyzer and to said first communication means, and adapted to request said second transceiver unit to shut off said carrier if the ratio of said signal component to said noise component is lower than a first pre-determined threshold,said second transceiver unit comprising:        a transmitter adapted to transmit over said physical channel a signal modulated over said at least one carrier,        a second communication means adapted to establish a communication link with said first transceiver unit,        a power control unit coupled to said transmitter and to said second communication means, and adapted to shut off said carrier upon a first request from said first transceiver unit.        
Such a pair of transceiver units is already known in the art, e.g. from the recommendation entitled “Asymmetric Digital Subscriber Line (ADSL) Transceivers-2 (ADSL2)”, ref. G.992.3, published by the International Telecommunication Union (ITU) in July 2002.
2. Related Art
FIG. 1 depicts a pair of transceiver units with a first Digital Subscriber Line (DSL) transceiver unit TU_C, housed in a Digital Subscriber Line Access Multiplexer (DSLAM) at a central office CO, and a second DSL transceiver unit TU_R, sited at customer premises CP, the transceiver unit TU_C being coupled to the transceiver unit TU_R via a twisted pair of copper wires L.
With respect to the direction of communication from the central office CO to the customer premises CP or downstream direction, the transceiver unit TU_C is a transmitter unit and the transceiver unit TU_R is a receiver unit. With respect to the direction of communication from the customer premises CP to the central office CO or upstream direction, the transceiver unit TU_R is a transmitter unit and the transceiver unit TU_C is a receiver unit.
Transceiver initialization is required in order for a physically connected pair of transceiver units to establish a communication link via a physical channel.
In order to maximize the throughput and reliability of this communication link, a transceiver unit shall determine certain relevant attributes of the physical channel and establish transmission and processing characteristics suitable to that channel.
Each receiver determines the relevant attributes of the channel by means of the transceiver training and channel analysis steps. Determination of channel attribute values and establishment of transmission characteristics requires that each transceiver produces, and appropriately responds to, a specific set of precisely-timed signals.
During a further data exchange step, each receiver shares with its peer transmitter certain transmission settings that it expects to see. Specifically, each receiver communicates the number of bits and relative power level to be used on each carrier.
Upon completion of the initialization procedure, the carriers are sorted as follows (see §8.6.4, p. 76-78):                Loaded carriers: these are the carriers for which the bit loading is greater than or equal to 1, that is to say the carriers used for communication.        Monitored carriers: these are the carriers for which the bit loading is set to 0, yet which keep on being transmitted. The purpose is to track the SNR and, in case the SNR improves, to use these carriers for communication.        Shut-off carriers: these are the carriers that are never used for communication, and for which the bit loading and the relative gain are both set to 0.        
A deficiency of the disclosed pair of transceiver units is that the shut-off carriers cannot be enabled again. This means that if the noise conditions change, the transceiver units have to stick with the carriers that are left.